Air contains small concentrations of the valuable inert gases helium, neon, argon, krypton, and xenon. Neon, argon, krypton, and xenon historically have been recovered as secondary products in the cryogenic separation of air into oxygen and/or nitrogen products. Argon, krypton, and xenon also can be recovered from the purge gas generated in the production of ammonia from synthesis gas generated from air and natural gas. Krypton and xenon are useful in a number of specialized fields including research, medicine, instrumentation, lighting, and space applications, and demand is expected to increase as these applications expand.
Because air contains only 1.14 and 0.087 ppmv of krypton and xenon, respectively, recovery of these components by the cryogenic separation of air is technically complex and costly. Recovery is complicated further by the presence of light hydrocarbons, particularly methane, in the air feed to air separation plants. Krypton, xenon, and methane, by virtue of their boiling points relative to oxygen and nitrogen, are concentrated in the liquid oxygen during the distillation process. In the separation of krypton and xenon from the liquid oxygen, the formation of flammable mixtures of methane and oxygen potentially can occur. Numerous processes have been developed in the air separation industry to circumvent this problem, but the processes are complex and safety is an ongoing concern in plant operation. Methods to recover krypton and xenon from air without the potential for forming flammable hydrocarbon-oxygen mixtures are therefore highly desirable.
The invention described below and defined by the claims which follow offers a method for recovering krypton and xenon from air without the potential to form such flammable hydrocarbon-oxygen mixtures in the krypton-xenon separation process. The method can be integrated with processes for the production and conversion of synthesis gas into liquid synthesis products wherein the synthesis gas is produced by partial oxidation of hydrocarbons with oxygen.
The invention relates to a method for recovering krypton and xenon from air which comprises:
(a) separating an air feed stream into oxygen-enriched and nitrogen-enriched product streams;
(b) reacting the oxygen-rich product stream with a hydrocarbon feed in a synthesis gas generation process to yield a synthesis gas stream comprising hydrogen, carbon oxides, krypton, and xenon, which synthesis gas stream contains essentially no oxygen;
(c) introducing the synthesis gas stream into a synthesis gas conversion process and converting the synthesis gas stream into a liquid synthesis product stream and an unreacted synthesis gas stream;
(d) recycling a portion of the unreacted synthesis gas stream to the synthesis gas conversion process; and
(e) separating another portion of the unreacted synthesis gas stream into a purge gas stream and a gas product stream enriched in krypton and xenon.
The hydrocarbon feed may be natural gas. The liquid synthesis product may comprise methanol or paraffinic hydrocarbons. The method may further comprise introducing steam into the synthesis gas generation process.
The separating of the remaining portion of the unreacted synthesis gas stream into the purge gas stream and the gas product stream enriched in krypton and xenon may be effected by one or more process steps selected from the group consisting of catalytic combustion, amine-based absorption, condensation and phase separation, pressure swing adsorption, temperature swing adsorption, and cryogenic distillation.
The method may further comprise reducing the pressure of the liquid product stream to yield a two-phase reduced-pressure product stream, and separating the two-phase reduced-pressure product stream into a final liquid synthesis product stream and a gas stream enriched in krypton and xenon. The method also may further comprise combining the gas stream enriched in krypton and xenon with another portion of the unreacted synthesis gas stream.
The purge gas stream may comprise at least hydrogen, carbon monoxide, and methane, and optionally a portion of the purge gas stream may be recycled to the synthesis gas generation process. Optionally a portion of the purge gas stream may be recycled to the synthesis gas conversion process. Another portion of the purge gas stream may be recycled to the synthesis gas generation process
In another embodiment, the invention relates to a method for recovering krypton and xenon from air which comprises:
(a) separating an air feed stream into oxygen-enriched and nitrogen-enriched product streams in an air separation process;
(b) reacting the oxygen-rich product stream with a hydrocarbon feed in a synthesis gas generation process to yield a synthesis gas stream comprising hydrogen, carbon oxides, krypton, and xenon, which synthesis gas stream contains essentially no oxygen;
(c) introducing the synthesis gas stream into a synthesis gas conversion process and converting the synthesis gas stream into a liquid synthesis product stream and an unreacted synthesis gas stream;
(d) recycling at least a portion of the unreacted synthesis gas stream to the synthesis gas generation process;
(e) reducing the pressure of the liquid synthesis product stream to yield a two-phase reduced-pressure product stream, and separating the two-phase reduced-pressure product stream into a final liquid synthesis product stream and a gas stream enriched in krypton and xenon; and
(f) separating the gas stream enriched in krypton and xenon into a purge gas stream and a gas product stream further enriched in krypton and xenon.
A portion of the unreacted synthesis gas stream may be combined with the gas stream enriched in krypton and xenon prior to (f). The hydrocarbon feed may be natural gas and the liquid synthesis product may comprise methanol. Alternatively, the liquid synthesis product may comprise paraffinic hydrocarbons. The method of may further comprise introducing steam into the synthesis gas generation process.
The separating of the remaining portion of the unreacted synthesis gas stream into the gas product stream enriched in krypton and xenon and the purge gas stream may be effected by one or more process steps selected from the group consisting of catalytic combustion, amine-based absorption, condensation and phase separation, pressure swing adsorption, temperature swing adsorption, and cryogenic distillation.
The purge gas stream may comprise at least hydrogen, carbon monoxide, methane, krypton, and xenon. A portion of the purge gas stream may be recycled to the synthesis gas generation process.
If desired, a portion of the purge gas stream may be recycled to the synthesis gas conversion process and optionally another portion of the purge gas stream may be recycled to the synthesis gas generation process. A portion of the purge gas stream may be recycled to the air separation process.